The present invention relates to a system and method of increasing the sulfation capacity of a sorbent and particularly to a system and method of increasing the sulfation capacity of limestone used to absorb sulfur dioxide produced during the combustion of coal in a fluidized bed combustor.
Removing pollutants from off gases is an important environmental consideration, and one method for removing or scrubbing unwanted sulfur dioxide from the flue gas or off gas produced during coal combustion is to contact same with an alkaline earth oxide for conversion thereof to the sulfate. As a result of the renewed interest in coal as a major energy source, considerable attention is being focused on the fluidized-bed combustion (FBC) of coal because the process offers potentially higher efficiency and lower capital cost than alternative processes. In the FBC process, crushed coal or other combustible material is burned at temperatures in the range of between about 700.degree. C. and about 900.degree. C. and at pressures in the range of between about 100 to 1000 kPa in a fluidized-bed of partially-sulfated, particulate, in situ sulfur dioxide sorbent such as limestone or dolomite. The bed material of limestone or dolomite transfers heat to the boiler tubes and reacts with and captures the potentially harmful sulfur dioxide released from the coal. Disadvantages of the process are the incomplete utilization of the calcium oxide in the calcined limestone which reacts with the sulfur dioxide present at elevated temperatures to form calcium sulfate with the resultant increase in the raw material cost and in the environmental impact of extensive quarrying and disposing of the large quantities of solid spent sorbent. Regeneration and reuse of spent limestone or dolomite, or alternatively an increase in the utilization of the available calcium oxide can reduce both the cost and environmental impact of this process.
Representative literature in the field includes the Strom et al U.S. Pat. No. 3,749,380 which discloses the use of a water slurry of calcium carbonate (5% solution) in a fluidized bed for reaction with sulfur dioxide gas. The reaction of calcium oxide and sulfur dioxide is not shown. The Robinson U.S. Pat. No. 3,751,227 discloses treatment of off gases with calcium carbonate particles wherein abrasive action of the particles removes the salt on the outside of the particles thereby exposing fresh calcium carbonate. The Wesselhoft et al U.S. Pat. No. 3,998,607 discloses an alkali metal catalyst useful in coal gasification and teaches the recovery thereof from reactor char by washing with water, thereby producing an alkali metal-rich aqueous solution. The Vogel et al U.S. Pat. No. 4,091,076 teaches the use of alkali metal or alkaline earth metal oxides impregnated with a refractory support material for use in the FBC process to scrub sulfur dioxide. Subsequent treatment of the sorbent material includes removal of the refractory support material and regeneration of the metal sulfate to the oxide by chemical reduction. The Hubble et al U.S. Pat. No. 4,081,522 teaches the regeneration of calcium sulfide formed during the fluidized-bed combustion of coal in the presence of limestone or dolomite. The material is subsequently reduced and treated with calcium sulfide to provide the requisite regeneration.